An increasing awareness of energy consumption and environmental issues as well as technology breakthroughs in the area of nanomaterial development has reignited an interest in the use of thermoelectric and photoelectric materials. Thermoelectric materials have properties that allow electricity to be generated from a temperature difference. Photoelectric materials are capable of collecting photons, preferably from a renewable source such as sunlight, and converting that energy into electricity. It is desirable to increase the energy conversion efficiency of these materials and lower the cost to produce thermoelectric and photoelectric materials. Due to the energy conversion efficiency and reduced cost that the present invention will enable, there is a potential to enable new applications of thermoelectric and photoelectric devices. In addition, there is a potential applicability for photocatalysts for hydrogen production, thermionic emitters, and application to fuel-cell membranes.
Thermoelectric Materials
Thermoelectrics is the science and technology associated with thermoelectric converters, that is, the generation of electrical power based on the Seebeck effect and refrigeration by the Peltier effect. The attractive features of thermoelectric devices are their long life time, low maintenance, lack of moving parts, capability to be miniaturized, lack of harmful environmental emissions, and their high reliability. Currently, thermoelectric generators are used to provide electrical power in medical, military and deep space applications where their desirable properties outweigh their relatively high cost and low operating efficiency. In recent years there also has been an increase in applications of thermoelectric coolers for use in infrared detectors, optical communications, and computing. The widespread use of thermoelectric components is presently limited by the low efficiency of presently known materials. The dimensionless thermoelectric figure of merit (ZT) of the best performing thermoelectric materials has remained around 1 for more than 50 years. Recent advances in nanotechnology have pushed the laboratory recorded ZT to 3.5. Currently these high ZT have only been achieved in laboratory scale materials and use very expensive processing techniques which make them unlikely candidates as low cost alternatives to simple waste heat recovery and heat management. Simultaneously, a significant amount of work has been initiated using processes that start with large nanoparticles and hot pressing them to result in a low cost material with room temperature ZTs of 1-1.4. The present invention will enable the production of high ZT materials (ZT>1) at a reduced cost.
To be an effective thermoelectric material, a compound must possess a large Seebeck coefficient, a low resistivity and a low thermal conductivity. Achieving these three properties simultaneously has proven to be a considerable challenge. Conventional thermoelectric materials are bulk solid solution alloys. Numerous bulk materials have been extensively studied for decades and the best known bulk solution alloys have room temperature figures of merit on the order of ZT≈1. It is generally acknowledged that this value of ZT is near the limit for bulk materials and thus, any improvement in ZT beyond this value is not likely.
This upper limit for bulk material ZT is due to the physical interrelationships between the Seebeck coefficient α, the thermal conductivity κ, and the electrical conductivity p. An increase in α is generally accompanied by an increase in the resistivity p because of carrier density changes. Furthermore, a decrease in the resistivity implies an increase in the electrical contribution to the thermal conductivity.
It has been demonstrated that highly efficient bulk thermoelectric materials can be produced through engineering material structures at the nanoscale. Although this nanoscale based thermoelectric material is highly efficient in terms of its energy conversion, the cost to produce these structures is prohibitive to all but a few niche market areas. The present invention produces bulk films of nanoscale materials at a low cost and with efficient properties in thermoelectric and, additionally, photovoltaic applications. Heat recovery in automobiles and nuclear power plants, general heating and cooling, and power generation are examples of applications for which thermoelectric technology could be utilized.
Photoelectric Materials
Renewable energy from the sun has great potential in reducing the dependency on fossil fuels while providing a cleaner, non-green house gas producing method of power generation. Photovoltaic (PV) devices that directly convert sunlight into electricity have found great acceptance in niche applications such as remote power for oil pipelines, monitoring stations and satellite power. A number of different device designs exist for photovoltaic cells alone including P-N and P-I-N single or tandem quantum dot (QD) junctions or hot carrier cells, intermediate band solar cells, dye sensitized cells (otherwise known as Gratzel cells), a variety of luminescent and luminescent concentrator cells, and extremely thin absorber (ETA) cells. Efficiency constraints associated with PV technology greatly limits its applicability as a wide-scale distributed power generation source.
Thus, in most electronic devices utilizing solar power, the limiting feature for overall system efficiency is the PV module efficiency. The PV module efficiency is dependent on the materials and processes used to create the module. Best in class crystalline silicon modules have materials with theoretical limits of 33% efficiency and in production as modules these devices have an efficiency of typically around 15%, thereby making final system efficiencies in the 10-13.5% range. Alternately, successful development of advanced materials with efficiencies approaching 60% that can be mass produced while minimizing the penalty on efficiency during production could result in systems with overall efficiencies in the 50-55% range, and thus yielding a four fold increase in available power for a fixed size module.
Semiconductor nanocrystals, otherwise known as quantum dots are nanometer-scale (nano-scale) structures that are composed of semiconductor materials. Due to the small size of the crystals (typically, 2-10 nanometers (nm)), quantum confinement effects manifest which results in size, shape, and compositionally dependent optical and electronic properties, rather than the properties of the bulk materials. Quantum dots have a tunable absorption onset that has increasingly large extinction coefficients at shorter wavelengths, multiple observable excitonic peaks in the absorption spectra that correspond to the quantized electron and hole states, and narrowband tunable band-edge emission spectra. Quantum dots absorb light at wavelengths shorter than the modified absorption onset and emit at the band edge.
The semiconductor nanocrystal complexes of the present invention can be adapted and then implemented into PV devices through solution phase self-assembly deposition on substrates and post processing techniques. These techniques are compatible with low-cost, large area metallized polymer substrates using roll-to-roll processing.
In contrast to the limitations of contemporary solar cell technologies, semiconductor nanocrystals, in particular colloidal semiconductor nanocrystals, allow for greatly increased solar cell efficiency as well as significantly decreased manufacturing costs. Because colloidal semiconductor nano crystals can be combined with polymers in solution, most solar cell research has focused on cells comprising semiconductor nanocrystals dispersed within conjugated polymers. Although this route can conceivably lead to low cost solar cells, the efficiency has been limited to a few tens of a percent to a few percent due to difficulties in facilitating charge transport through the quantum dot/conjugated polymer interface. In addition, charge transport is compromised due to voids between the colloidal particles. The present invention enables the production of a bulk film at substantially lower costs than the present PV production methods, with said bulk film exhibiting high PV energy conversion primarily due to filling of the voids between micron-scale composite particles of II-VI, III-V, IV-VI, IV, tertiary, quaternary, and quinary semiconductors with nanocrystals and nanocomposites. Nanocomposites are typically formed by the bonding of a nanocrystal with at least one hydrazine ligand and/or at least one nanoentity. The nanoentity is selected from a group comprising nanoparticles, nanorods, nanotetrapods, and nanowires.